The emerging GSM-EDGE and UMTS standards for mobile telecommunications place an increasingly stringent requirement on the linearity of handsets, particularly given their proposed wider channel bandwidths. In order to realise a power-efficient handset design, some form of linearisation will be required in the handset transmitter which should be (i) low-power itself; (ii) capable of broadband linearisation (up to 5 MHz for UMTS/ULTRA; (iii) frequency flexible, and preferably multi-band; and (iv) capable of achieving and maintaining high-levels of linearity improvement with highly-non-linear power amplifiers (e.g. class-C).
This increasing dynamic range requirement, also has an effect on the RF front-end systems of both handsets and base stations. In the case of handsets, the use of software radio techniques, with the requirement for adaption between many different channel and system bandwidths over a wide range of operating frequencies, leads to a desire to remove RF front-end filtering. The alternative solution of switchable or tuneable filters either results in poor filter performance or an unrealistic number of separate filters. Without a front-end filter, a wide range of signals, both very strong and very weak, can enter the receiver. These must be processed (i.e. amplified and down converted) without undue distortion that might produce unwanted intermodulation products on the wanted channel that mask the wanted signal. Thus, a front-end with a good noise figure and a high intercept point is required.
In the case of base-station systems, these are typically designed for a single frequency band, but are required to operate in a harsh radio environment, with the transmitters from other, unrelated systems being sited close by (often on the same mast). This results in large unwanted signals impinging upon the receiver system, which signals have been removed using large and expensive filtering.
The invention is also applicable to adaptive antenna systems. Such systems are currently implemented using RF gain and phase-shifting components (often in a “Butler Matrix” configuration) and this arrangement requires these components to operate at high power levels and is therefore less than optimum in performance. A new and better method of implementing the gain and phase shifting elements involves the processing of all signals from a base-station (in a composite multi-carrier form) at baseband. The gain and phase controls can then be implemented easily (e.g. in a digital signal processor DSP) and this will become increasingly attractive as DSP technology improves. However, to be successful, there is a requirement for very high linearity multi-carrier upconversion and power amplification.